10d los angeles smog.scripts
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A&O2TRANSCRIPT
One of the main secondary pollutants formed in Los Angeles smog is ozone. While it is the same molecule one should not confuse ozone at the surface with ozone in the stratosphere, which protects us from the sun’s UV radia?on. There is much more ozone in the stratosphere and the ozone at the surface cannot take over this role. On the other, hand ozone at the surface is a concern as this is where humans live. High levels of ozone can cause eyes to tear and can make it difficult to breath. To have an idea how bad ozone levels in Los Angeles were and s?ll are we need to look at what we believe the natural concentra?ons of ozone are. We know very liFle about ozone in the distant past, but we have one piece of informa?on from Paris around the year 1900. At Montsouris a group of scien?st made accurate measurements of ozone which researchers today were able to reanalyze to determine the ozone mixing ra?os. 120 years ago there were around 5 – 15 ppb of ozone in Paris. Since this was before the widespread use of the combus?on engine we believe that these were the pre-‐industrial levels of ozone. If one looks in remote areas today the background levels of ozone are more in the range of 30-‐40ppb. The global surface ozone levels have thus more than doubled in the past 120 years. We can now compare this value with those measurement in Los Angeles in the 1970’s which reached up to 600ppb, 20 ?mes higher than the natural levels. Even today we can, on occasion, reach mixing ra?os of up to 150ppb. It is not uncommon to find such high mixing ra?os in other urban areas around the world.
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Here is a model calcula?on on the Average July aZernoon ozone levels around the world. We again can see the high ozone mixing ra?os coincide with the most heavily populated areas in the work, such as the US, Europe, and southeast Asia. We can also see that the biomass burning area in southern Africa has some high ozone. The ozone life?me in the troposphere is around 1 month. Ozone can thus be transported quite effec?vely between con?nents. For example one can see the ou^low from Asia into the Pacific as well as the influence of North America on the Atlan?c.
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This figure shows the spa?al distribu?on of ozone in the LA Basin in 1988. In contrast to the primary pollutants the highest ozone levels can be found on the east side of Los Angeles and in the surrounding mountains. The West-‐side of Los Angeles has some of the lowest ozone levels. Later we will discuss this behavior in more detail, but for now we can summarize it as the interplay of the slow inland transport of Los Angeles pollu?on and the chemical forma?on of ozone along the way. Ozone mixing ra?os have steadily decrease in Los Angeles, as shown here in this figure from the California Air Resources Board. The black line shows the maximum one hour ozone value in a year, whicle the gray curve shows the 4th highest 8-‐hour average ozone mixing ra?o, a common measure to track the longterm trend of ozone in a city. Both measures have decreased since 1992. The decrease in ozone has slowed down since the year 2000 and the improvements are now quite slow. Nevertheless, we now rarely see ozone above 150ppb in Los Angeles, and averaged over an 8 hour period the highest values are more in the 100pb range.
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Another way of looking at this improvement is to compare the number of days various areas in LA exceeded the na?onal air quality standard in a year, here shown as a 3 year average. While all areas in the LA basin encountered some days above the na?onal air quality standard and the en?re east side of the basin had more than 100 days above the standard. In 2011 much of LA had 0 – 5 days above the air quality standard and only the most eastern part of the basin s?ll had more than 100 days of exceedances.
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